(1) Field of the Invention
The present invention relates generally to pin tumbler-type cylinder locks and associated keys. More particularly, this invention is directed to enhancing the security afforded by cylinder locks and especially to enhancing the ability of such locks to resist operation by a key obtained from an unauthorized source. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
(2) Description of the Prior Art
U.S. Pat. No. 4,823,575 discloses a novel cylinder lock and an associated key. The patented key, in addition to conventional bitting, is provided with a pair of projections on the opposite edges of the blade. These projections perform the dual function of operating an auxiliary pin tumbler stack, to permit rotation of the key and plug relative to the shell, and retaining the key in the keyway. The unique manner of implementation of the key retention function also reduces key and lock wear when force is imparted to the inserted key to move the door in which the lock is installed. U.S. Pat. No. 4,823,575 depicts a key wherein the bitting is in the form of depressions formed in the key blade, i.e., the patent depicts a "Kaba" type key. It will be understood by those skilled in the art, however, that the patented invention is equally applicable to a key wherein the bitting is, again by way of example only, in the form of conventional serrations in one or both edges of the key blade. Additionally, the patent depicts a reversible key, i.e., a key wherein the bitting is symmetrical and the two projections are of substantially the same size and shape. It will also be obvious to those skilled in the art that the patented invention is applicable to a key and lock combination where the pin tumbler arrangement is not symmetrical and where the projections serve separate and discrete functions, i.e., one projection operates the auxiliary pin tumbler stack while the other projection is intended for key retention only.
The lock and key of U.S. Pat. No. 4,823,575 have enjoyed significant commercial success. The patented combination, however, lacks a capability which many purchasers of high security locks desire. This capability is known in the art as "trapping". With trapping capability, should an attempt be made to operate a lock with an unauthorized key, such key is captured in the lock if the attempt to rotate the plug relative to the shell proves successful. A "trapped" key may not be withdrawn from the keyway without disassembly of the lock and trapping will prevent further plug rotation.